Method for synchronizing signals in a terrestrial and satellite link and associated architecture

Abstract

A method for synchronizing the transmission of messages in a communication system comprises a user terminal, a main communication network comprising a first station, and a secondary communication network comprising a second station, the method consisting of an exchange of synchronization signals to calculate an offset to be applied to the instant of emission of the messages transmitted by the second station so that the messages transmitted by the first station and the second station are received by the user terminal in a synchronous manner. A communication system implementing the invention is provided.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This application claims priority to foreign French patent application No. FR 1402969, filed on Dec. 23, 2014, the disclosure of which is incorporated by reference in its entirety.FIELD OF THE INVENTION[0002]The invention lies in the field of telecommunications, and more particularly in the field of telecommunications systems dedicated to the control and command of drones.[0003]It is aimed at proposing a method making it possible to synchronize the instants of arrival of communications originating from a satellite network and from a terrestrial wireless network and bound for a user terminal.[0004]The invention describes the architecture of a satellite / terrestrial hybrid transmission system implementing the method.BACKGROUND[0005]The insertion of unmanned aircraft, or drones, into the airspace is currently limited to appropriate, so-called “segregated”, spaces so as to limit the risks of incidents. Segregated spaces are reserved spaces, ofte...

AI Technical Summary

Benefits of technology

[0026]The invention therefore consists in proposing a synchronization method making it possible to slave all of the temporal references of the equipment of a communications network to that of the satellite station, and making it possible to calculate an offset to be applied to the transmissions from the terrestrial station. The aim of the method consists in that the signals originating from the satellite station and from the terrestrial station reach the user terminal in a synchronous manner.

[0027]Thus, the implementation of the method allows the simultaneous use of the satellite link and of the terrestrial link, in a communications system using access modes of TDMA or WCDMA type, or transmitting simultaneously on the two links by using waveforms having lesser capabilities for tolerating propagation delays than those of OFDM.

Problems solved by technology

The insertion of unmanned aircraft, or drones, into the airspace is currently limited to appropriate, so-called “segregated”, spaces so as to limit the risks of incidents.

The opening up of non-segregated airspace to drones poses numerous technological problems, such as that of the reliability of anticollision mechanisms, or that of the reliability of the telecommunication systems used for control and command.

The drawback of such a solution is that the system is unavailable during the failure detection time and during the time for re-establishment of the communication via the secondary network.

Depending on the duration of service interruption, the failure can be considered to be a loss-of-service-continuity event.

Moreover, depending on the redundancy mechanism implemented, the packets currently being transmitted via the main network may be lost and will have to be retransmitted by the secondary network.

The failure will therefore lead to more considerable transmission latency.

The latency jitter will therefore also be more considerable.

In this case the re-establishment of the communications is not transparent to the users.

As all the users of the main network are impacted by the failure, the communications re-establishment mechanism will cause a spike in usage of the network in respect of the protocol exchanges.

Depending on the dimensioning of the system, this spike may give rise to a choking of the system and in all cases will lead to a more considerable latency.

This strategy is therefore not suitable for the control and command of drones.

The drawback of this second solution is that in nominal mode it requires twice as much bandwidth over the air and in the networks.

However, this mechanism is not suitable for access modes requiring accurate synchronization between the signals, such as TDMA (Time Division Multiple Access), or WCDMA (Wideband Code Division Multiple Access).

Moreover, the delay between the signal transmitted by the satellite network and the signal transmitted by the terrestrial network may moreover be considerable.

This delay is compatible with the use of OFDM modulation, but not necessarily with waveforms having more limited possibilities for compensating for delays, such as waveforms with temporal equalizer, or spread waveforms associated with Rake receivers.

However, such operation is incompatible upon the addition of a second network, since this would make it necessary to apply two different time shifts at the level of the user terminals.

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